A method for preparing high-pressure lithium iron phosphate cathode material and the prepared lithium iron phosphate cathode material

Abstract

This invention discloses a method for preparing high-compact lithium iron phosphate (LFP) cathode material and the prepared LFP cathode material. Using lithium, iron, phosphorus, and carbon sources as raw materials, the LFP cathode material is prepared via a high-temperature solid-state method. A flux is added to the raw materials, and the flux is selected from one or more of magnesium carbonate, magnesium iodide, and magnesium hydroxide. This invention uses magnesium-containing compounds such as magnesium carbonate, magnesium iodide, and magnesium hydroxide as fluxes, enabling the preparation of LFP cathode material under low-temperature sintering conditions. This significantly reduces energy consumption, effectively avoids the generation of magnetic foreign matter such as iron phosphate, reduces electron transport resistance, and promotes particle densification growth. The compacted density of the prepared LFP cathode material is not less than 2.467 g / cm³. 3 The resistivity is not higher than 32.5 Ω•cm.

Description

Technical Field

[0001] This invention relates to a method for preparing cathode materials for lithium-ion batteries, and more particularly to a method for preparing high-pressure lithium iron phosphate cathode materials. Background Technology

[0002] With the rapid development of human society and the increasing demand for resources, the consumption of traditional petrochemical resources such as coal and oil has led to increasingly serious environmental problems, severely impacting people's lives. Therefore, environmentally friendly clean energy is widely used as an alternative. Compared to other types of secondary batteries, lithium-ion batteries are widely used due to their advantages such as high energy density, high voltage, no memory effect, long cycle life, wide operating temperature range, and environmental friendliness. In lithium-ion batteries, the cathode material is the high-value-added material with the highest technological barriers and the highest cost. Currently commercialized lithium-ion battery cathode materials include lithium cobalt oxide, lithium manganese oxide, ternary materials such as lithium cobalt-nickel-manganese oxide, and lithium iron phosphate. With the rapid development of the new energy vehicle market, lithium iron phosphate materials, due to their low price, non-toxicity, good safety performance, and long lifespan, are rapidly developing and possess huge market potential worldwide as the cathode material for lithium-ion batteries.

[0003] Currently, the main synthesis methods for lithium iron phosphate (LFP) cathode materials for lithium-ion batteries include high-temperature solid-state synthesis, co-precipitation, sol-gel, and Pechini methods. Among these, the co-precipitation, sol-gel, and Pechini methods, which are soft chemical methods, are complex and difficult to industrialize. High-temperature solid-state synthesis is the most commonly used method in conventional production. LFP is generally prepared using the high-temperature solid-state method, specifically by mixing and grinding a lithium, phosphorus, iron, and carbon source to obtain a precursor, which is then sintered at high temperatures. The sintering temperature for synthesizing LFP materials is relatively high (around 700–800℃ or even higher). This high sintering temperature places high demands on the sintering furnace equipment, increases energy consumption, and reduces the lifespan of the equipment. Furthermore, it increases the risk of magnetic foreign matter (such as iron phosphide) formation, degrading battery performance. However, lowering the sintering temperature would reduce the compaction density of the LFP material.

[0004] Therefore, how to achieve the same or better compaction effect while lowering the sintering temperature is an urgent problem to be solved. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing high-density lithium iron phosphate cathode material, which can effectively reduce the sintering temperature required for synthesizing lithium iron phosphate material and can produce lithium iron phosphate cathode material with low magnetic impurity content and high density.

[0006] The technical solution adopted by the present invention to solve its technical problem is: a method for preparing high-pressure solid-state lithium iron phosphate cathode material, which uses lithium source, iron source, phosphorus source and carbon source as raw materials, and obtains lithium iron phosphate cathode material by high-temperature solid-state method, wherein a fluxing agent is added to the raw materials, and the fluxing agent is selected from one or more of magnesium carbonate, magnesium iodide and magnesium hydroxide.

[0007] Furthermore, the flux is magnesium carbonate.

[0008] Furthermore, the amount of flux added is 0.4% to 0.8% of the mass of the iron source.

[0009] The core innovation of this invention lies in the precise control of the amount of magnesium-containing flux added to be 0.4% to 0.8% of the iron source mass. This ratio range is the optimal range obtained by the inventors through extensive experimentation. It is the key to achieving the synergistic effect of multiple technologies such as low-temperature sintering, high-pressure solid density, low resistivity, and absence of magnetic foreign matter. The application of this specific ratio of magnesium-containing flux in the preparation of lithium iron phosphate is not mentioned in the prior art, nor has any technology been found to provide technical inspiration for this ratio range. The selection of this ratio is not a conventional attempt by those skilled in the art and has significant inventiveness.

[0010] From the perspective of reaction mechanism, the ionic radius of Mg²⁺ in magnesium-containing fluxes (magnesium carbonate, magnesium iodide, magnesium hydroxide) is similar to that of Fe²⁺. With an iron source mass ratio of 0.4% to 0.8%, Mg²⁺ can be doped into the interstitial spaces of lithium iron phosphate in small amounts and uniformly, rather than forming lattice substitution or excessive impurity phases. This doping method can, on the one hand, play a nucleation induction role during the growth of lithium iron phosphate particles, reducing the activation energy for the formation of lithium iron phosphate crystal phases, thereby effectively reducing the sintering temperature to 600-700℃, avoiding the reduction reaction of Fe²⁺ and P at high temperatures, and fundamentally eliminating the generation of magnetic foreign matter such as iron phosphate. On the other hand, the uniform distribution of trace amounts of Mg²⁺ can form conductive bridges between lithium iron phosphate particles, reducing the electron transport resistance between particles, while promoting the densification growth of particles, so that the compaction density and conductivity of the material form a positive synergy.

[0011] If the flux addition is less than 0.4% (e.g., 0.1% in Comparative Example 4), the Mg²⁺ doping amount is insufficient, failing to effectively reduce the activation energy for crystal phase formation. This results in a weak effect on lowering the sintering temperature and makes it difficult to form continuous conductive bridges. Although the resistivity is low, the material's compaction density is only 2.447 g / cm³, failing to meet the high-compaction technical requirements of this invention and thus unable to satisfy the high-density molding requirements of lithium-ion battery cathode materials. If the flux addition is greater than 0.8% (e.g., 1% in Comparative Example 5), excessive Mg²⁺ doping... The g²⁺ content exceeds the tolerance limit of the lithium iron phosphate lattice interstices, forming a free magnesium salt impurity phase. Although this impurity phase further increases the compaction density of the material (2.556 g / cm³ in Comparative Example 5), it becomes an obstacle site for electron transport, leading to a sharp increase in the resistivity of the material (41.3 Ω・cm in Comparative Example 5), which significantly degrades the electrochemical conductivity of lithium iron phosphate. At the same time, excessive magnesium salts adsorb onto the surface of lithium iron phosphate particles, affecting the lithium ion insertion / extraction efficiency and reducing the cycle performance and rate performance of the battery.

[0012] The 0.4% to 0.8% iron source mass ratio specified in this invention represents the optimal balance range for lattice doping effect, nucleation-induced effect, and conductive bridging effect. This can be clearly verified by combining the test data in Table 1: Example 1 (0.4%) and Example 2 (0.6%) serve as typical cases within this range. The compaction densities of the prepared lithium iron phosphate cathode materials reached 2.467 g / cm³ and 2.475 g / cm³, respectively, both not less than 2.467 g / cm³, meeting the high compaction requirements; the resistivity was 15.3 Ω·cm and 18.5 Ω·cm, respectively, far below the upper limit of 32.5 Ω·cm, indicating excellent conductivity; and the iron phosphide content was 0 ppm in both cases, with no magnetic foreign matter generated. Compared to Comparative Examples 4 and 5, which had unbalanced proportions, the proportion range of this invention achieves optimal compaction density and resistivity. Compared to Comparative Examples 1 and 2, which did not use flux, at a low temperature of 600–700°C, it avoids the formation of iron phosphide caused by high temperature (comparative Example 1: iron phosphide 526 ppm) and solves the problem of a sharp drop in compaction density caused by low temperature sintering (comparative Example 2: compaction density only 2.066 g / cm³). At the same time, compared to other types of flux (comparative Example 3: NaPO3F), it shows significant technical advantages in both compaction density and resistivity.

[0013] Furthermore, 1% to 3% of a dispersant by mass of the iron source is added to the raw material. The dispersant may be polyethylene glycol, citric acid, PVP, or PVDF.

[0014] Furthermore, the mass of the carbon source is 8% to 12% of the mass of the iron source.

[0015] Furthermore, the addition of magnesium-containing flux at 0.4%–0.8% of the iron source mass exhibits good technical compatibility with the dispersant, carbon source ratio, and wet / dry grinding, sintering, and pulverizing processes of this invention. The 0.4%–0.8% flux addition synergistically works with the 1%–3% dispersant by the iron source mass to improve the uniformity of raw material grinding, ensuring uniform distribution of Mg²⁺ in the precursor. Simultaneously, it complements the 8%–12% carbon source by the iron source mass. The carbon source provides a conductive layer on the particle surface, while the flux provides conductive bridging between particles. The combination of these two components further reduces the material resistivity. The uniformly distributed Mg²⁺-induced densified particles maintain good particle morphology even after subsequent air jet milling, ensuring excellent formability of the powder with a D50 particle size of 1.0–1.2 μm, ultimately yielding a high-pressure lithium iron phosphate cathode material with excellent overall performance.

[0016] Furthermore, the high-temperature solid-state method involves adding flux and solvent to the raw material for wet grinding, spray drying the obtained wet-ground material, and sintering the resulting granulated powder under a protective atmosphere to obtain lithium iron phosphate cathode material; or adding flux to the raw material for dry grinding, and sintering the obtained dry-ground material under a protective atmosphere to obtain lithium iron phosphate cathode material.

[0017] Furthermore, the sintering temperature under the protective atmosphere is 600-700℃ and the sintering time is 8-10h. The protective atmosphere is N2 or Ar as the protective gas, and the oxygen content is less than 10ppm.

[0018] Furthermore, the raw materials are wet-milled by adding flux and solvent, and the particle size is controlled to be 0.40-0.42 μm. The obtained wet-milled material is spray-dried and granulated, and the particle size of the spray-dried granules is controlled to be 27-30 μm and the spray-drying outlet temperature is 100-110℃. The obtained granulated powder is sintered under a protective atmosphere to obtain lithium iron phosphate cathode material.

[0019] Furthermore, the lithium iron phosphate cathode material is pulverized by an air jet mill, with the grading frequency controlled at 50-60Hz, the feeding frequency at 10-20Hz, and the required D50 particle size after pulverization to be 1.0-1.2μm.

[0020] A lithium iron phosphate cathode material, wherein the lithium iron phosphate cathode material is prepared by any of the above-described high-pressure lithium iron phosphate cathode material preparation methods.

[0021] The beneficial effects of this invention are as follows: This invention uses magnesium-containing compounds such as magnesium carbonate, magnesium iodide, and magnesium hydroxide as fluxes, enabling the sintering of lithium iron phosphate cathode materials under low-temperature conditions. This significantly reduces energy consumption, effectively avoids the generation of magnetic foreign matter such as iron phosphide, reduces electron transport resistance, and simultaneously promotes the densification and growth of particles. The resulting lithium iron phosphate cathode material has a compaction density of not less than 2.467 g / cm³. 3 The resistivity is not higher than 32.5 Ω•cm. Detailed Implementation

[0022] The present invention will be further described below with reference to embodiments.

[0023] Example 1:

[0024] A method for preparing a high-pressure lithium iron phosphate cathode material includes the following steps:

[0025] (1) According to the molar ratio Li:Fe:P = 1.033:1:1.035, the lithium source, iron source and phosphorus source are initially mixed, wherein the lithium source is lithium carbonate, and the iron phosphate is the iron source and phosphorus source. The iron-phosphorus ratio of the iron phosphate is required to be 0.960~0.970; then water is added to obtain an aqueous solution, and the solid content of the aqueous solution is adjusted to 37%.

[0026] Then, polyethylene glycol (a dispersant), glucose (a carbon source), and magnesium carbonate (a flux) are added sequentially to the aqueous solution to obtain a mixture; wherein the mass of polyethylene glycol is 2% of the mass of ferric phosphate, the mass of glucose is 10% of the mass of ferric phosphate, and the mass of magnesium carbonate is 0.4% of the mass of ferric phosphate.

[0027] (2) The above mixture is homogenized and stirred for 1 hour, the water bath temperature is controlled at 30-40℃, and then wet grinding is carried out, with the grinding particle size controlled at 0.40-0.42μm;

[0028] (3) Spray dry the obtained wet-milled material to granulate it. Control the spray-drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃. Sinter the obtained granulated powder at 650℃ for 9 hours under nitrogen atmosphere. Then, pulverize the sintered material by air jet mill. Control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and the required D50 particle size to be 1.0-1.2 μm after pulverization to obtain lithium iron phosphate cathode material.

[0029] Example 2:

[0030] A method for preparing high-pressure lithium iron phosphate cathode material, which differs from Example 1 in that the mass of magnesium carbonate is 0.6% of the mass of iron phosphate, thus obtaining lithium iron phosphate cathode material.

[0031] Example 3:

[0032] A method for preparing a high-pressure lithium iron phosphate cathode material, which differs from Example 1 in that the mass of magnesium carbonate is 0.8% of the mass of iron phosphate.

[0033] Example 4:

[0034] A method for preparing a high-pressure lithium iron phosphate cathode material, which differs from Example 2 in that the flux is magnesium iodide.

[0035] Example 5:

[0036] A method for preparing a high-pressure lithium iron phosphate cathode material, which differs from Example 2 in that the flux is magnesium hydroxide.

[0037] Comparative Example 1:

[0038] A method for preparing a high-pressure lithium iron phosphate cathode material, which differs from Example 1 in that magnesium carbonate is not added and the sintering temperature is 800℃.

[0039] Comparative Example 2:

[0040] A method for preparing a high-pressure lithium iron phosphate cathode material, which differs from Example 1 in that magnesium carbonate is not added.

[0041] Comparative Example 3:

[0042] A method for preparing a high-pressure lithium iron phosphate cathode material, which differs from Example 1 in that the flux is NaPO3F.

[0043] Comparative Example 4:

[0044] A method for preparing a high-pressure lithium iron phosphate cathode material, which differs from Example 1 in that the mass of magnesium carbonate is 0.1% of the mass of iron phosphate.

[0045] Comparative Example 5:

[0046] A method for preparing a high-pressure lithium iron phosphate cathode material, which differs from Example 1 in that the mass of magnesium carbonate is 1% of the mass of iron phosphate.

[0047] Table 1

[0048]

Claims

1. A method for preparing a high-pressure solid-state lithium iron phosphate cathode material, comprising using lithium source, iron source, phosphorus source, and carbon source as raw materials, and preparing the lithium iron phosphate cathode material by a high-temperature solid-state method, characterized in that: A flux is added to the raw materials, and the flux is selected from one or more of magnesium carbonate, magnesium iodide, and magnesium hydroxide.

2. The method for preparing a high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that: The flux is magnesium carbonate.

3. The method for preparing a high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that: The amount of flux added is 0.4% to 0.8% of the mass of the iron source.

4. The method for preparing a high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that: The raw material contains 1% to 3% by weight of a dispersant, which may be polyethylene glycol, citric acid, PVP, or PVDF.

5. The method for preparing a high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that: The mass of the carbon source is 8% to 12% of the mass of the iron source.

6. The method for preparing a high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that: The high-temperature solid-state method involves adding flux and solvent to the raw materials for wet grinding, spray drying the resulting wet-ground material, and sintering the resulting granulated powder under a protective atmosphere to obtain lithium iron phosphate cathode material; or adding flux to the raw materials for dry grinding, and sintering the resulting dry-ground material under a protective atmosphere to obtain lithium iron phosphate cathode material.

7. The method for preparing a high-pressure lithium iron phosphate cathode material according to claim 6, characterized in that: The sintering temperature under the protective atmosphere is 600-700℃ and the sintering time is 8-10h. The protective atmosphere is N2 or Ar as the protective gas and the oxygen content is less than 10ppm.

8. The method for preparing a high-pressure lithium iron phosphate cathode material according to claim 6, characterized in that: The process involves adding flux and solvent to the raw materials for wet grinding, controlling the grinding particle size to be 0.40–0.42 μm, spray drying the obtained wet-ground material to granulate it, controlling the spray-dried granulation particle size to be 27–30 μm and the spray drying outlet temperature to be 100–110 °C, and sintering the obtained granulated powder under a protective atmosphere to obtain lithium iron phosphate cathode material.

9. The method for preparing a high-pressure lithium iron phosphate cathode material according to claim 6, characterized in that: The lithium iron phosphate cathode material was pulverized by an air jet mill, with the grading frequency controlled at 50-60 Hz, the feeding frequency at 10-20 Hz, and the required D50 particle size after pulverization to be 1.0-1.2 μm.

10. A lithium iron phosphate cathode material, characterized in that: The lithium iron phosphate cathode material is prepared by the high-pressure lithium iron phosphate cathode material preparation method according to any one of claims 1 to 9.

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